VAKIATION IN VIRULENCE 229 



1. Variation by Mutation 



Although random mutation is usually invoked as the basis for variation in 

 animal viruses and, on this basis, mutation rates have been calculated 

 (Burnet and Bull, 1943; Medill-Brown and Briody, 1955; Dulbecco and Vogt, 

 1958), in only one instance has there been any attempt to demonstrate the 

 essential character of a mutational change — namely, its randomness 

 (Dulbecco and Vogt, 1958). In this instance, the d to d + mutation of polio- 

 virus, it was possible to show significant fluctuation in the number of mutants 

 present in sister populations of virus grown in aliquots of one cell population. 

 In other words, sister populations of virus show nonrandom variation in 

 proportion of mutants if multiplication has occurred since they were separated, 

 whereas the variation is, of course, random if there has been no multiplication 

 in the interim. 



2. Host-Induced Variation 



There are at least two examples of what is probably host-induced variation 

 of an animal virus. The first, and less conclusive, concerns the behavior of 

 unadapted influenza A virus and Newcastle disease virus (NDV) in mouse 

 brain; after small inocula of non-neurotropic influenza virus into mouse brain, 

 there is a single cycle of virus production, the virus produced being infective 

 for the allantois but apparently not for mouse brain in that multiplication 

 there was confined to one cycle (Cairns, 1951, 1954). The second concerns the 

 behavior of encephalomyocarditis virus in mouse brain; a variant of this 

 virus, adapted to form plaques on sarcoma 180 cells, loses its capacity to form 

 plaques on these cells if passed through a single cycle in mouse brain; this 

 capacity is restored after a single cycle in Krebs 2 carcinoma cells (Sanders 

 and Hoskins, 1958). 



3. Genetic Recombination 



The genetic importance of sex in higher organisms, as a means of producing 

 novel combinations of characters and providing the opportunity for rapid 

 spread of mutant characters through the gene pool, needs no emphasis. The 

 very short generation time and enormous populations of viruses provide a 

 much greater opportunity for the combined interplay of mutation (random or 

 host-induced) and selection to serve as the major basis for evolution. 



Except possibly with bacteriophages it seems to us unlikely that genetic 

 recombination is of importance in the evolutionary history of viruses, but 

 recent work has shown that it may be a useful method for producing novel 

 combinations of virus characters in the laboratory. In a number of animal 

 viruses only phenotypic mixing has been detected, i.e., the novel combination 

 dissociates to the parental types on passage. Genetic recombination has been 

 unequivocally demonstrated with influenza virus (Burnet, 1955) and with 



